US2555520A - Core mounting for permeability tuners - Google Patents

Description

June 5; 1951 A, J, TORRE 2,555,520'

CORE MOUNTING FOR PERMEABILI'IY TUNERS Filed June 1, 1948 l Ey lo co/vpucr//va MA TEE/A L 'Z4 VM/ M A M :inventor ALTDN Tm-1N Tunne- Gttomeg Patented June 5, 1951 CORE MOUNTING FOR PERMEABILITY TUNERS Alton John Torre, Westmont, N. J., assigner to Radio Corporation of America, a corporation of Delaware Application June 1, 1948, Serial No. 30,485

(Cl. Z50-40) 6 Claims.

This invention relates generally to permeability tuning systems, and particularly a multi-range permeability tuner including a paramagnetic core connected so as to minimize the capacitor losses of the tuner at high frequencies.

It is frequently desired to provide a broadcast receiver adapted to receive modulated waves within two or more different frequency ranges. Thus, for example, it is conventional practice to provide a broadcast receiver for the reception of both amplitude-modulated (AM) and frequencymodulated (FM) carrier Waves. In that case, the two tuning ranges of the receiver are between approximately 500 to 1600 kilocycles (kc.) and 88 to 106 megacycles (mc). It is also conventional practice to effect tuning' of a receiver by moving a paramagnetic core relatively to a tuning inductance coil forming part of a resonant circuit thereby to vary the inductance of the coil. A paramagnetic material is defined as a material having a magnetic permeability greater than that of a vacuum, which is unity. The magnetic permeability of a paramagnetic material may be independent of the magnetizing force or it may vary with the magnetizing force, in which case the material is usually called ferromagnetic.

It is well known that distributed capacitance exists between a paramagnetic core and its coil. When the core is moved into and out of the coil for varying the resonant frequency of a circuit including the coil, the amount of capacitance between core and coil will vary. At high frequencies this distributed capacitance has an appreciable reactive impedance which is quite undesirable because its magnitude varies throughout the tuning range. Furthermore, the distributed capacitance causes capacitive losses across the coil which will increase the power factor of the resonant circuit and thus decrease the Q of the circuit. On the other hand, within a low frequency range the reactive impedance of this distributed capacitance becomes negligible. At such a low frequency range, however, it is desirable to maintain the core at a fixed potential such as ground potential. In most cases, the permeability tuner of a superheterodyne receiver comprises a signal frequency circuit and an oscillator tank circuit, each of which may be tuned by a separate core. In that case, the two cores should be electrically decoupled which may be effected by connecting both of them to the same xed potential.

It is an object of the present invention, therefore, to connect a paramagnetic core arranged for tuning several tuning coils in succession, in such a manner that the undesirable distributed capacitance between the core and at least one of the coils is neutralized, thereby to reduce the capacitive losses across the coil.

Another object of the invention is to provide a permeability tuner including a high frequency tuning coil and a core for tuning the coil, the ccre being connected in such a manner as to minimize the capacitive losses across the coil.

A further object of the invention is to provide a multi-range permeability tuner of the type where a single paramagnetic core is movable to tune in succession two tuning coils and where the core is maintained at a fixed potential within a low frequency range and is allowed to have a floating radio-frequency potential within the high frequency range.

A conventional permeability tuning system may comprise a low frequency range tuning coil and a high frequency range tuning coil and a single core movable relatively to the coils for tuning them in succession. In accordance with the present invention there is provided a choke coil, such as a helically Wound wire, for connecting the core to a fixed potential point such as ground to which the permeability tuning system is also connected. The choke has negligible impedance within the low frequency range and accordingly the core is effectively grounded at all frequencies within the low frequency range. For frequencies within the high frequency range the choke has an appreciable impedance and hence will permit the core to have a floating radio-frequency potential, that is, it may assume the high frequency potential of its associated coil. In this manner the capacitance existing between the core and the high frequency ccil may be neutralized.

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawing, in which:

Fig. 1 is a schematic view, partly in section, of a permeability tuner embodying the present invention;

Fig. 2 is a schematic circuit diagram of the high frequency range tuning coil and the paramagnetic core grounded through a choke;

Fig. 3 is an equivalent circuit diagram for the lovv frequency range of the tuning coil where the core is effectively directly grounded; and

Fig. 4 is a circuit diagram of a multirangc oscillation generator in which the permeability tuner of the invention may be used with advantage.

Referring now to Fig. 1 there is illustrated a permeability tuner comprising high frequency range inductance coil i6 and low frequency range nductance coil II. As will be more fully explained in connection with Fig. e, coils Iii and II form part of two separate resonant circuits tunable over different frequency ranges such as.

used in a superheterodyne receiver arranged for receiving both AM and FM waves. Coils iii and II preferably are both wound on the same cylindrical coil form I2 which may be made of a suitable insulating material. Paramagnetic core i3 is axially movable within coil form I2fso as-to tune coils It and II in succession. 1t is also feasible to use a diamagnetic core, that is, a core having a permeability smaller than unity, however the core i3 must in all cases be enough electrically conductive so that it may be effectively grounded, as will be hereinafter explained.

Referring now to Fig. 2, high frequency tuning coil IB and core I3 are illustrated schematically.

Coil I0 may be resonated by capacitor E4 and may have one terminal grounded as illustrated. Resonant circuit IB, I4 accordingly may be tuned over a predetermined high frequency tuning range by moving core I3 with respect to coil Iii. As is well known, distributed capacitance, represented partially by capacitors I5, exists between core I3 and each turn of coil I5. The total effective distributed capacitance varies in magnitude with the movement of core I3 with respect to the coil. This capacitance is very undesirable because it increases the power factor of resonant circuit Iii, I4 and hence decreases the Q of the circuit. The power factor of a resonant circuit is inversely proportioned to the Q of the circuit which *i* may be defined as the energy stored by the circuit divided` by the energy dissipated per cycle of the resonant frequency of the circuit. In accordance with the present invention this capacitance is isolated from ground by connecting an electrical choke I6 between core I3 and ground as shown in Fig. 2. This will permit core I 3 to have a oating radio-frequency potential. Accordingly, the capacitive losses which appear across coil i0 are minimized.

As illustrated in Fig. 1, choke I6 may take the form of a tightly wound spiral electrically conductive wire having one end connected to core I3. The other end of choke I6 is connected to adjustable support I'I which is rigidly connected to core I3 by a non-conducting rod I8 which preferably consists of a non-magnetic material. Adjustable support Il is grounded as indicated schematically. Accordingly, core i3 is connected to ground through a choke having an appreciable reactive impedance within a predetermined high frequency range.

In order to tune resonant circuit Ill, I4, core I3 `must be moved with respect to coil I il. A mechanism for moving core I3 has been shown for convenience schematically only and may comprise a threaded rod 2] which is threaded through movable support I? and guided by bearings 2 I, 22. Bearings 2|, 22 are secured to base plate 23. Threaded rod 20 may be rotated, for example, by handle 24, thereby to move support I7 in the direction of arrows 25. Movement of support Il will cause a corresponding movement of core I3 through the intermediary of rod i8. Choke I5 and switch 55.

will therefore not change its shape when core I3 is moved.

Let it now be assumed that core I3 is moved towards the right of Fig. 1 into low frequency tuning coil II. Referring now to Fig. 3, there is illustrated coil il and core I3. Capacitor 26 is connected across coil II to resonate it within a predetermined low frequency range. One terminal of coil II may be grounded as shown. It will now be assumed that Within the low frequency range the distributed capacitance between coil Il and core i3 may be neglected. In that case, it is no longer necessary to ground core I3 through a choke and preferably core I3 is directly grounded as illustrated. This may be effected by making the reactive impedance of choke I6 so small within the low frequency range that it may be disregarded. Accordingly, core I3 is effectively directly grounded for frequencies within the low frequency range to which resonant circuit il, 26 may be tuned, while the core is isolated from ground by a choke for frequencies within the high frequency range to which resonant circuit l2, I4 may be tuned.

In a superheterodyne receiver, there is usually provided a tunable signal frequency circuit as well as a tunable oscillator tank circuit. Both circuits may be tuned by separate cores. When both cores are grounded they are thereby decoupled which is very desirable.

Referring now to Fig. 4 there is illustrated a two range oscillator circuit of the general type disclosed and claimed in the copending application to Torre and Kirkwood, Serial No. 17,174,

filed on March 26, 1948, and entitled Frequency Converter and Oscillator Circuit. The oscillation generator of Fig. 4 is arranged to develop waves with two separate frequency ranges and comprises triode 46 including cathode 4I, control grid 42 and anode 43. The oscillation generator further comprises oscillator 'tank circuit 44 including coil Ill and capacitor I4. Oscillator tank circuit 44 preferably is tunable Within a high frequency range of approximately 98 to 118 mc., for receiving FM waves. Tank circuit 44 has one terminal grounded while its other terminal is coupled to control grid 42 through coupling condenser 45 bypassed by grid leak resistor 46. The high frequency oscillation generator further includes tickler coil 41 inductively coupled to coil Iii and connected between ground and cathode 4I through coupling condenser 48. Anode 43 is connected to a suitable source of voltage indicated at -l-B through `dropping resistor 50. The junction point between dropping resistor and anode 43 is bypassed for alternating currents through bypass condenser 5 I.

The oscillation generator, as described so far, operates in a conventional manner. The grid circuit includes the oscillator tank circuit 44, and the required feedback to the cathode is provided through tickler coil 41. Coupling capacitor 48 preferably has a small reactance within the high frequency range to which tank circuit 44 may 4be tuned.

The oscillation generator may further be arranged to oscillate within a low frequency range between approximately 1 and 2 mc. for the reception of AM waves. For this purpose there is provided another oscillator tank circuit 53 con sisting of coil II and capacitor 26. One terminal of tank circuit 53 is grounded while the other terminal may be selectively connected to control grid 42 through coupling capacitor 54 Another tickler coil 56 is connected between cathode 4I and ground and inductively coupled to coil Il.

When switch 55 is closed, the oscillation generator will oscillate at a frequency determined by tank circuit 53. Grid leak resistor 46 is grounded through a low impedance path including coil IU. Tuning coils l0 and Il may be tuned by core I3 shown schematically adjacent to each coil, although in operation it will be understood that core I3 actually will be associated with only one coil at a time, depending upon the frequency to be received. As illustrated in Fig. 1, tickler coils 41 and 56 may be wound over high frequency range coil l0 and low frequency range coil Il, respectively. It is to be understood, however, that the permeability tuner of the invention as illustrated in Fig. 1 may also be used with other circuits, the oscillation generator of Fig. 4 being shown by way of example only.

There has thus been described a multi-range permeability tuner having a single core for tuning coils and where the core may be directly grounded within a low frequency range and grounded through a choke withinahigh frequency range to neutralize the distributed capacitance between the core and its associated coil. Thus, the capacitive losses across the coil are minimized, the power factor reduced and the Q irnproved.

What is claimed is:

l. In a permeability tuning system having a fixed potential connection, a high frequency range tuning coil, an electrically conductive core movable relatively to said coil for tuning it, and

means providing a direct-current path including an unbypassed choke for connecting said core to said fixed potential connection, thereby to isolate the capacitance existing between said coil and said core, by raising the radio frequency potential of said core above ground potential at frequencies within said high frequency range.

2. In a permeability tuning system having a fixed potential connection, a low frequency range tuning coil, a high frequency range tuning coil, an electrically conductive core movable relatively to said coils for tuning said coils in succession, and means providing a direct-current path including an unbypassed choke for connecting said core to said fixed potential connection, said choke having negligible impedance within said low frequency range, and high impedance within said high frequency range.

3. In an electrical tuning system, a first resonant circuit tunable over a high frequency range and including a first inductance element, a second resonant circuit tunable over a low frequency range and including a second inductance element, a core movable for tuning said inductance elements in succession, and a high frequency choke connected electrically between said core and a point of fixed potential with respect to said circuits, said choke having negligible im- Cal Til

pedance within said low frequency range, and a high impedance within said high frequency range, thereby to essentially isolate the capacitance existing between said core and said rst inductance element within said high frequency range only. l

4. In a multi-range permeability tuning system, a first resonant circuit tunable over a high frequency range and including a rst inductance element, a second resonant circuit tunable over a low frequency range and including a second inductance element, each of said circuits including a low impedance ground connection, a core movable for tuning said inductance elements in succession, and a wire helically wound in a fixed position with respect to said core connected electrically between said core and said ground connection, said wire having negligible impedance within said low frequency range and functioning as an electrical choke within said high frequency range, thereby to isolate the capacitance existing between said core and said first inductance element within said high frequency range only.

5. In an unbypassed permeability tuning system, a core movable to tune a plurality of tuning inductance elements for low and high frequency ranges in succession, and a choke coil conductively connecting said core to a point of xed potential with respect to said system and electrically floating said core at frequencies above said low frequency range, said choke coil having negligible impedance Within said low frequency range and high impedance within said high frequency range.

6. In a permeability tuning system, a core movable to tune a plurality of tuning inductance elements for low and high frequency range in succession, said core having secured thereto in a fixed position relative and movable therewith a helically wound wire for connecting said core to a point of fixed potential with respect to said elements, said wire functioning as an electrical choke within said high frequency range to isolate the capacitance existing between said core and said tuning inductance elements within said high frequency range only.

ALTON JOHN TORRE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,850,914 Bodoh et al Mar. 22, 1932 2,106,226 Schaper Jan. 25, 1938 2,137,435 Yolles Nov. 22, 1938 2,320,483 Stocker June 1, 1943 FOREIGN PATENTS Number Country Date 382,076 Great Britain Oct. 22, 1932 447,104 Great Britain May 12, 1936

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